Idle revs too high or too low - Cleaning the Idle Air Control Valve

Common causes for the idle revs being erratic and running either too fast or too slow are failure of the engine coolant temperature (ECT) sensor or the Idle Air Control valve (IACV), commonly referred to as the Stepper Motor. The latter suffers from a build-up of dirt, which over time, can lead to it jamming or partially blocking the valve.

Let me give you some idea of the problem I was experiencing. The engine would always start on the first attempt and although the fault did not always manifest itself, it would only become noticeable once the coolant temperature reached it's normal running level. Some days there would be nothing wrong, but on others it would start to loose power below 2000 rpm and would possibly cut out altogether when the revs dropped below 1000 rpm. I would have to keep the revs above 1500 rpm or higher just to be able to pull away from being stationary. Once the engine had been allowed to cool down for an hour or so, the whole process would begin again, but might run perfectly okay. Over a period of several weeks, this got progressively worse, so that in the end it was happening on every journey.

Having replaced, plugs, leads, distributor cap and rotor to no avail, and with the problem clearly linked to temperature, it became obvious that it had to be either the ECT or the IACV.

( NOTE: A sticking throttle flap can also be a cause. Don't be tempted to use an adjustable Jubilee Clip to secure the rubber hose from the air filter to the throttle housing. The standard housing is made of plastic, and excessive tightening of such a clip can distort the housing and lead to a sticking throttle flap. Use a fixed diameter clip instead. See here )

The IAC valve is mounted on the inlet manifold and controlled by the ECM. The IAC valve opens a pintle ("bolt in a hole") valve situated in an air passage in the throttle housing, allowing air to bypass the throttle disc and flow straight into the inlet manifold.

By changing the amount the IAC valve is open, the ECM can control engine idle speed and cold start airflow requirements by adjusting the flow of air in the passage.

To quote from the Service Manual...."During cold starting the ECM indexes the IAC valve open slightly to provide a level of fast idle, dependent on engine coolant temperature. As the engine warms, fast idle is gradually decreased until normal operating temperature is reached. The position of the IAC valve can be checked using TestBook and should be within the range of 20 to 40 steps when the engine is running. If it is identified as being outside this range it can be adjusted to within range using TestBook. This ensures that the IAC valve is at the optimum position within its range for providing further movement to compensate for changes in engine load or temperature in accordance with signals from the ECM."

On engine switch off, the ECM will set up the IACV for the next engine start.

First, here's what it looks like.........

	The stepper motor and valve when removed from the engine............
	...........and separated.
	After 13 years of use, this was how dirty mine was !
	After cleaning, this is what it should have looked like.
	Similarly, you can see that the valve port also needed cleaning.

Now, how to get it off the vehicle..........

Release the hood clamps above the windscreen, unzip the rear windscreen, unclip the 5 clamps, and fold the rear of the hood forward, being careful not to crease the plastic window and secure the hood out of the way with a length of string.

Remove the parcel shelf trim and the sound deadener pad.

Remove the 11 bolts securing the engine cover and remove the cover.

Inside the boot, release the 4 retaining clips along the front edge of the Engine Compartment Access Panel, and remove the Panel

	The stepper motor is here.
	To give better access to the lower fixing screw, use a pair of pliers to release the pipe retaining clip and pull off the pipe from the engine head, and then from the manifold.
	Then depress the connector-retaining clip (part of mine has broken away!) and pull off the connector and move it aside.
	Then undo and remove the top torx screw.
	The lower torx screw is harder to see and more likely to be dropped! I suggest that you use a long-reach driver. Locate it in the screw head and as you undo it, use your other hand to pull and lift the stepper motor up and away from the engine, while retaining the screw still in the motor flange. You are far less likely to loose the screw somewhere down in the bowels of the engine bay. Use the same principle in reverse when refitting the motor. Remember that the torx screws are fitting into plastic - the recommended torque value is only 1.5Nm !

The other half, i.e. the valve port, is awkward to remove and I would suggest that if you can, it is better to clean it in situ, rather than struggling to remove it. Note that all the screws, both for this and the stepper are fitting into plastic and therefore the less disturbed, the better.



Now, clean it up and use a little WD40 to lubricate it. It is possible to test it at this point.

	I would suggest that before you test the stepper motor, you should measure the amount that the 'bolt' sticks out of the body. Testing will alter this length and since this is the position last determined by the ECU, it is probably the best starting position when refitting the motor if it passes the following test. After adjusting and measuring mine, I have the impression that when the valve is fully closed, this measurement is between 28mm and 29mm, and it's normal running position when the engine is hot is therefore between 25mm and 28mm, though I cannot guarantee this.
	As the circuit diagram and notes state, it is possible to test the stepper motor by applying 12v to Pin 2 and sequentially connecting GND to Pins 1, 3, 4, and 6. Circuit courtesy of Tom White, Switzerland
	To extend the bolt outwards follow the sequence of Pins 4, 6, 1, 3. You can repeat this sequence over and over again to move it further, but don't do it too far. You should feel a step movement each time you make contact with a pin, though if the motor is already in the position relative to the pin connector you initially make contact with, there will be no movement. On subsequent loops around the sequence, it should cause movement. To retract the bolt inwards follow the sequence of Pins 3, 1, 4, 6. You can also measure the resistance across each circuit. Mine measured 8.8, 8.8, 8.7, and 8.8 ohms. If the movement and resistance is consistent, then the stepper would appear to be working okay, and I would suggest that you 'drive' back so that it is left in the same position as when you measured it earlier.

Now to check the Engine Coolant Sensor

	Easily distinguished since it is coloured brown.
	Remove the spring clip from the connector using long-nosed pliers...........
	......and set clip aside. Then remove multi-plug connector.
	Before removing the sensor, you can make a quick check with a multi-meter across the pins to see if the sensor has gone open circuit - i.e. no connection. If you are going to replace it immediately, there will be a small amount of coolant loss. You will need a 19mm spanner to undo it. Wait until the engine is cold, so that the thermostat is closed (it will reduce the coolant flow), and have your thumb ready to block the hole. Ideally you should replace the washer on the sensor with a new one (it is not supplied with the sensor!), but the existing washer should do the trick if need be. Remember to later check for leaks once the engine is back up to running temperature.
	Temp       Resistance 0            4.8K-6.6K 10          4.0k 4.0k 20          2.2k-2.8k 30          1.3k 1.3k 40          1.0k-1.2k 50          1.0k 1.0k 60          800 800 80          270-380      Circuit courtesy of Tom White, Switzerland

Checking the resistance can be a bit deceptive. With a cold engine, mine gave a 2.4k reading; warm at 27.5C gave 1.73k; and at 60C the value had dropped to 336 Ohms, so it was responding to a certain degree (pun not intended!).

With both the IACV and ECT seeming function, but the problem still existing, I opted to replace the ECT sensor, since this was by far the cheapest option; it's measured resistance at the higher temperature was half of what it should be; and the stepper motor appeared to work quite satisfactorily.

When refitting the sensor, tighten to 6 Nm.

Now, if a failing ECT sensor has been affecting the control of the stepper motor, the latter's 'saved' position stored in the ECU might be wayout when coming to restart the engine after replacing the sensor. There is a way to reset the valve (see below), but it has to be done when the engine is at it's normal operating temperature. You might find that the engine will not start because once you turn the key to start it, the ECU will reposition the valve to an incorrect and very open position.

Do not continuually attempt to restart the engine if this is happening, because all this will do is pump neat petrol through the engine and into the CAT, which will not do it any good.

There are several options..........

Get somebody to squeeze the rubber hose that feeds into the IACV, thereby effectually closing the valve while you start the engine.

Remove the hose from the valve and block the valve orifice. Then start the engine.

In either case, keep the revs up and run the engine until it reaches it's normal operating temperature.

Once the temperature is reached, turn the engine off, refit the hose if need be, and follow the procedure below.

IACV resetting proceedure

Start the engine and run it until normal operating temperature is reached.

Turn off the ignition.

Turn the ignition on to position II (but do not start the engine)

Press the accelerator pedal fully down and release it 5 times in succession.

Wait for at least 15 seconds and then turn the ignition off.

Start the engine and the IACV stepper motor should be reset.

For further information of sensors have a look at Dieter's or Tom White's web sites.